2016 AMC 8 Problems/Problem 3
Four students take an exam. Three of their scores are and . If the average of their four scores is , then what is the remaining score?
We can call the remaining score . We also know that the average, 70, is equal to . We can use basic algebra to solve for : giving us the answer of .
Since 90 is 20 more than 70 and 80 is ten more than 70, for 70 to be the average, the other number must be thirty less than 70, or .